The invention relates to an apparatus for forming an extended nip between a mating roll and a shoe roll for treating a product web, in particular a paper web.
Apparatuses for forming extended nips press sections and calender installations of papermaking machines. The lubrication between a rotating roll shell and a stationary press shoe is preferably provided purely hydrodynamically at normal operating speeds. Such hydrodynamic lubrication between the roll shell and the press shoe is produced by lubricating oil being applied upstream of the press shoe, relative to the machine running direction, and being drawn into a drawing-in region by the rotating roll shell and to build up a hydrodynamic lubricating film between the roll shell and the press shoe.
Such hydrodynamic lubrication has the advantage that the pressing pressure profile in the machine running direction from the inlet region of the press shoe initially increases continuously and then, towards the end of the shoe over a relatively short region, decreases to zero. Pressure profiles of this type have process engineering advantages during the dewatering operation in press sections and the calendering operation. A disadvantage of such purely hydrodynamic lubrication is that the lubricating film between shoe and shell is no longer reliably produced, is deficient or is not brought about at all at low speeds and high line loads, i.e., high surface pressures. Such deficient lubrication can lead to mixed friction between shoe and roll shell. External indications of such friction are initially high drive powers or drive torques. The consequence of mixed friction is, firstly, wear phenomena, in particular on the roll shell, and, secondly, high heating of the roll shell, of the press shoe and of the lubricating oil. This can lead to destruction of the roll shell.
These disadvantages in the operating range of low speeds and high line loads can be avoided by additional hydrostatic lubrication, such as oil pockets, with additional lubricating oil introduced into the running surface of the press shoe. Such a static supply of lubricating oil ensures the lubricating function under all operating parameters, but necessitates greater expenditure in construction and fabrication and, in particular, places high requirements on the pressurized oil control. For this purpose, DE 43 11 622 A1 discloses providing the shoe in its surface with recesses, that is to say zones of different spacing from the mating roll, in the machine running direction, the said recesses being used to build up a hydrostatic pressure cushion but being free of any external feed and discharge lines. The revolving shell or the circulating belt drags lubricating oil into these recesses, so that, in the region of these zones, the lubricating oil film therefore has a greater thickness. As a rule, the oil dragged in remains in these zones, so that, at a low operating speed, at which the oil film of a hydrodynamic shoe becomes smaller, since less oil is carried along by the revolving shell, breakdown of the lubricating film can be avoided to the greatest possible extent in the region of the pockets. However, the disadvantage in this case is the influence produced by the recesses on the pressure profile, and also the dependence of the functional ability of the hydrostatic lubrication on the hydrodynamic lubrication.
It is therefore an object of the invention to provide an apparatus for forming an extended nip which permits reliable lubrication in all speed and line force ranges.